1. Field of the Invention
The invention relates to a method and apparatus for the dispensing of natural gas. More particularly, the invention provides a method and apparatus for filling a storage tank with natural gas and removing natural gas from this storage tank for dispensing to a user, such as a natural gas consuming vehicle.
2. Description of the Related Art
Due to increasing environmental concerns, vehicle manufacturers are under pressure to convert to motor fuels that produce lower amounts of pollutants. Carbon dioxide, a byproduct of the combustion of hydrocarbon fuels, is now considered as a pollutant insofar as this gas contributes to a "hot house effect" which is expected to significantly raise the general temperature of the Earth's environment with potentially disastrous consequences. One method of reducing the production of carbon dioxide has been to use hydrocarbon fuels that have a higher proportion of hydrogen relative to carbon. Natural gas is such a fuel. Therefore, there has been increasing pressure, due to environmental concerns, to convert entire fleets of vehicles so that they can consume natural gas rather than gasoline or diesel fuel.
Under usual conditions, natural gas is supplied by pipeline under low pressure to storage facilities. At the storage facility, the natural gas is compressed, using large compressors, into large storage vessels at pressures up to about 4,000-5,000 psi. Natural gas can then be dispensed from these large storage vessels into the fuel tanks of vehicles. However, as natural gas is removed from the storage vessel, the gas pressure in the vessel declined. As the pressure in the vessel declined, the rate of discharge of natural gas from the large pressure vessel to the fuel tank also declined, especially since the vehicle fuel tanks must be filled to a pressure of at least about 2,400 psi or even higher. Therefore, storage vessels have so far as known only been capable of supplying gas at a declining rate of flow, as the pressure gradually dropped and approached the vehicle tank pressure. This declining and overall relatively slow rate of filling user vehicles has been a significant impediment to the conversion of vehicles to the use of natural gas since filling a vehicle under these conditions could take several hours.
In order to overcome the slow filling rate obstacle, "fast-fill cascade" systems have been developed. However, these systems also have limitations. In a fast-fill cascade system, natural gas has been supplied via a compressor to a series of compressed gas storage cylinders. These storage cylinders have been initially charged at high pressure, typically about 3,600 psi. The vehicle requiring a refill of fuel was hooked up to one of these compressed gas cylinders. Once the gas in that particular one of the cylinders dropped below a certain pressure or required feed rate, the cylinder was taken out of service. A second cylinder in the series was then brought into service to provide fuel. This cylinder operation was designed to be carried out automatically by controlled valves so that there was a relatively continuous flow of fuel to the vehicle. However, fast-fill cascade systems of this type have been expensive, requiring large numbers of high pressure gas cylinders and associated valving and controls. Further, in order to fuel a large fleet of vehicles, such as the buses for a large metropolitan area transit system, a vast number of fast-fill cascade high pressure cylinders would be required. This would require a substantial capital investment. The same is true for natural gas service stations selling gas to the general public who require fast service (rapid fueling rates). This situation is made worse by the fact that the capital investment is not normally economically justifiable, but is necessitated by environmental concerns. Gasoline or diesel fuel is virtually always cheaper.
There has existed a need for a capability of rapidly fueling vehicles with compressed natural gas (CNG) or vapors from liquid natural gas (LNG) with a short fueling time per vehicle. Further, the means should desirably require relatively low capital investment while meeting safety and environmental standards.